Seismic sources used in land geophysical exploration of subsurface formations have, so far as is known, been of two general types. The first type has been explosives which were usually detonated in shot holes during seismic surveys. In addition to the hazards present in dealing with explosive substances, there were several other problems. One was the inability to repeat successive shots from the same shot point. Another was the inability to control the amplitude and frequency content of the signal generated on explosion.
A second type of seismic source involved the use of counter-rotating or reciprocating weight masses. When the weight masses were mechanically caused to move, the weight and size needed in the systems to impart adequate energy caused a problem. The requisite bulk rendered the systems quite heavy, cumbersome and difficult to move or transport. Additionally, there were practical limits on the bandwidth of the seismic signal which could be formed. These relatively large bodies could not be induced to move or rotate beyond certain speeds or frequencies. When the weight masses were caused to move by hydraulic or pneumatic fluids, the foregoing problems were still present. An additional problem that arose was that of the heat induced in the fluids, which then had to be dissipated.